Refining element for a refining disc

ABSTRACT

Refining elements for refining discs for disc refiners for the manufacture of mechanical pulp, for example, board pulp, are subjected to hard abrasive wear in acid environment and at high temperature. In order to improve the wear resistance alloys with precipitated carbides are used. A service life improved in comparison with known alloys is obtained with a refining disc with the following analysis in % by weight: 2.96 C, 0.77 Si, 0.82 Mn, 24.2 Cr, 5.16 V, 0.04 Ni, 0.03 Mo, and the remainder Fe and impurities. After casting, the refining segments are hardened and annealed and assume a hardness of 57-63 HRC.

TECHNICAL FIELD

[0001] This invention relates to refining elements for a refining discfor disc-refiners intended for the manufacture and/or treatment offibrous pulps, where the refining element is produced by casting a steelalloy and hardened and heat treated to a hardness of at least 55 HRC.

BACKGROUND OF THE INVENTION AND KNOWN STATE OF ART

[0002] Disc-refiners for the refining of lignocellulosic material, i.e.for the mechanical manufacture or treatment of so-called mechanicalpulp, are known, for example, through SE 506 822 and 402 019. Theycomprise two circular refining discs, which are rotated relative to eachother, and have refining surfaces built-up of refining elements(normally called refining segments), which comprise bars and grooves andguide the pulp from the centre out to the periphery during the refiningoperation. The refining surfaces are subjected to heavy abrasive weardue to foreign hard particles, sand, in the chips. The temperature,besides, is high, often about 220° C., and the wood yields an acid pulpwith a pH, which at the making of newsprint is about 6.5, but at boardmanufacture is as low as 4-5, which requires corrosion resistance. Inorder to reduce the wear, alloys with precipitated carbides are used.

OBJECT OF THE INVENTION AND DESCRIPTION OF THE INVENTION

[0003] The invention has the object to provide refining elements of theaforedescribed kind, which have an improved service life. This object isachieved in principle in that the refining disc has the followinganalysis, in per cent by weight: 2.7-3.2 C, 0.5-1.0 Si, 0.7-1.2 Mn,21.0-26.0 Cr, 3.0-6.0 V. at maximum 0.5 Ni, at maximum 0.5 Mo, and theremainder Fe and impurities.

[0004] Vanadium is a very strong carbide former with a considerablygreater affinity to carbon than chromium and the vanadium carbide has ahardness, which clearly exceeds that of the chromium carbide. Already atthe solidification a precipitation of vanadium carbides is obtainedwhich improves both the wear resistance at abrasive wear and thecorrosion resistance. The last-mentioned property is understood andexplained in that every per cent of vanadium binds up to 0.23% carbon.As a result thereof the carbon content in the matrix decreases incorresponding degree, which has the consequence that the carbon contentavailable for chromium for the formation of carbides becomes lower. Thechromium carbides, which are precipitated, contain also a certain amountof vanadium. Therefore, the proportion of chromium substitution-solvedin the matrix, which improves the corrosion resistance, increases. Dueto the adapted contents of carbon, chromium and vanadium the primaryprecipitated carbides assume a desired size, so that the tenacity is notreduced thereby that the primary precipitated carbides are too great.The fracture surfaces of the alloys according to the invention areconsiderably more fine-grained than of the other chromium-alloyedcasting alloys for refining elements. A material, thus, is obtainedwhich has both improved resistance to abrasive wear and improvedcorrosion resistance. This is important especially for refiningsegments, which are intended to be used for the refining of board pulp.

[0005] In order to achieve sufficient hardness of above 55 HRC, usually57-63 HRC, after hardening and heat treatment, the hardenability must besufficient. Therefore, the carbon content must be kept high, and if thestated analysis interval for the carbide forming elements Cr, Mo and Vand for Ni are exceeded, the hardness will not be achieved. If thecarbon content exceeds the stated analysis interval, the carbides growand embrittle the material.

[0006] If the lower limits for Cr and V are fallen short of, the desiredmixture of carbide types essential for the wear resistance is notobtained.

[0007] In order to achieve maximum wear resistance and tenacity, thematerial must be hardened and annealed in a conventional way. Inconnection with this heat treatment a secondary carbide fraction isprecipitated, which is more finely dispersed than the one obtained atthe solidification.

DESCRIPTION OF A PREFERRED EMBODIMENT

[0008] An alloy according to the invention has been cast and heattreated and compared with two known alloys used for refining segments.The anlyses are shown in the following table, which also shows theresults of the wear tests. Table of chemical composition in percent byweight and results of wear tests Wear Alloy C Si Mn Cr V Mo Ni (mg)Alloy acc. 2.96 0.77 0.82 24.2 5.16 0.04 0.03 170 to inventionComparison 1.12 0.89 0.93 16.70 0.19 0.61 0.13 260 alloy 1 Comparison2.63 0.43 0.75 26.60 0.35 0.02 0.10 200 alloy 2

[0009] In order to be able to evaluate and rank the properties of thealloys, an abrasive wear test is used where a definite sample size ofthe metal samples is worn against grinding paper for a definite periodand with a constant pressure. The tests were made in water of roomtemperature. The abrasive wear resistance was measured as weight loss inmilligram. Of each alloy three sample pieces were made, and four testsof each sample piece were carried out. The mean values are shown in theTable above. The lower the weight loss, the greater, thus, is theresistance to abrasive wear. The result cannot be directly convertedinto expected service life, because several parameters such as pH,temperature, rotation speed of the refiner, a.o. have an effect underthe operation conditions. The test in laboratory environment has theadvantage that one is not subjected to all imaginary variations arisingat use in operation. As a completion full-scale tests in operation werecarried out with refining segments of the alloy according to theinvention, and a similar test with refining segments of comparison alloy1. The refining segments had exactly the same surface pattern. The alloyaccording to the invention had a service life which was 80% better thanthat of the comparison alloy.

1. A refining element for a refining disc for disc refiners for themanufacture and/or treatment of fibrous pulps, where the refiningelement is produced by casting a steel alloy and hardened and heattreated to a hardness of at least 55 HRC, characterized in that therefining element has the following analysis in % by weight: 2.7-3.2 C,0.5-1.0 Si, 0.7-1.2 Mn, 21.0-26.0 Cr, 3.0-6.0 V, maximum 0.5 Ni, maximum0.5 Mo, and the remainder Fe and impurities.
 2. A refining element asdefined in claim 1, characterized in that the content of C is 2.8-3.1%by weight.
 3. A refining element as defined in any one of the precedingclaims, characterized in that the content of Si is 0.7-1.0% by weight.4. A refining element as defined in any one of the preceding claims,characterized in that the content of Cr is 22.0-25.0% by weight.
 5. Arefining element as defined in claim 4, characterized in that thecontent of Cr is 23.0-24.5% by weight.